Uranium and other metal values can be recovered from commercial grade wet process phosphoric acid by liquid-liquid extraction processes. In these processes, phosphoric acid solution is contacted, generally in a multistage, counter-current extractor, with an organic extractant solvent composition having an affinity for uranium values. After extraction, two phases are formed, namely an aqueous phase and an organic phase rich in uranium values. Then, the organic phase is stripped of its uranium content, and the stripped organic solvent is returned to the extraction system.
McCullough, in U.S. Pat. No. 2,830,872 and McGinnis, in U.S. Pat. No. 2,835,552, recognized that the organic extractant solvent composition can lose its uranium extraction efficiency after about 7 passes through the extraction-stripping system. The uranium extraction coefficient, E.degree., which should be high, i.e. at least about 2.0, and preferably between about 4.0 to 5.0, was found to decay from an initial value of 6, to a decayed value of about 1.0 to 1.5. This extraction decay requires either increasing the number of equilibrium stages in the extractor, increasing the solvent to aqueous ratio, or discarding the solvent, which usually contains many expensive components. In a similar area, Sundar et al., in U.S. Pat. No. 3,966,872, recognized that the uranium-rich extractant solvent also contained ferric ions which could be removed by water scrubbing the entire stream after extraction but before stripping, so that amine extraction in subsequent cycles was not interfered with.
McCullough solved his extraction decay problem by heating the stripped organic solvent to 60.degree. C. to evolve hydrofluoric acid, followed by treating the solvent with solid phosphorous pentoxide, and then washing the treated solvent with 60% orthophosphoric acid prior to recycle to the extraction system. McGinnis, in solving a similar type of problem, washed the solvent with 5% sulfuric acid to strip hydrofluoric acid from the solvent, prior to solvent recycle to the extraction system. While the McCullough and McGinnis processes are effective to varying degrees, a simpler, less expensive method of treatment is needed to solve solvent extraction decay problems.